This invention relates to the production of synthetic polyamides and, more particularly, to a continuous process for producing polyamides by the polymerization of polyamide-forming aqueous salt solutions.
Synthetic polyamides are important commercially, particularly those which exhibit film or fiber-forming properties. Such polyamides are the linear condensation products of diamines and dicarboxylic acids, which are usually prepared by heating an aqueous salt solution prepared from a selected diamine and a selected dicarboxylic acid, with the removal of water until the desired degree of polymerization is attained.
Film or fiber-forming polyamides may be prepared batchwise or continuously. In general, polyamides may be prepared more economically, more uniformly and with less handling by a continuous process. In some continuous processes, an aqueous solution of polyamide-forming reactant, e.g., a diaminedicarboxylic-acid salt, is supplied continuously to an elongated tube having a predetermined elevated inlet pressure, e.g., 15 atmospheres, and passed through the elongated tube wherein a temperature sufficiently high for polymerization to occur (and above the melting point of the resultant polyamide) is maintained and the pressure is gradually reduced along the tube to progressively flash water from the reaction mass as steam. Such an elongated tube is frequently referred to in the art as a "flasher". The reaction mass is then passed continuously to another heated vessel frequently referred to as a "finisher".
In other continuous processes, the aqueous solution of polyamide-forming reactants is first fed continuously to a reactor wherein the temperature and pressure conditions are such that the salt is partially polymerized before it is passed continuously through a flasher to a finisher.
In any of the above-continuous processes in which flashers are used, problems may be encountered in maintaining a substantially constant level in the vessel (finisher) which follows the flasher. These problems in maintaining substantially constant level become greater as the deviation in instantaneous "rate in" vs. "rate out" increases. This deviation in flow is caused by instabilities associated with two phase (liquid-vapor) flow in the flasher. These problems in level control (which are often present in steady state operation as a result of small fluctuations in pressure, temperature, viscosity, etc.) are aggravated by changes in throughput, such as those which occur in fiber production if one or more spinning positions are shut down or started up. In fiber production, a substantially constant level in the finisher can make the difference between a system in which it is difficult to make a product with good dye uniformity and one that is capable of consistently producing dye critical product. The reason for this is that variations in residence time in the system affect the final degree of polymerization obtained and hence the dye pick up on the dye receptive end groups in the polymer. J. A. Carter et al. in Canadian Pat. No. 800,061, issued Nov. 26, 1968, discloses a method of maintaining a substantially constant level in the vessel following the flasher during changes in monomer throughput which involves feeding a composite aqueous solution of polyamide-forming reactants to the flasher; the composite being formed from two separate streams of different concentration and the rates of flow of the two streams being varied such that the rate of introduction of water is substantially constant.
The main disadvantage of the above method for maintaining a substantially constant level in the vessel which follows the flasher are that it is cumbrous and that it is not effective in continuous processes in which the aqueous solution of polyamide-forming reactants are first fed to a vented reactor before being passed to a flasher.
Surprisingly, it has now been found that the above disadvantages of such prior art processes may be overcome and a substantially constant level in the vessel following the flasher may be maintained by controlling the pressure in the flasher by injecting steam at an intermediate point in the flasher while controlling a single feed stream to the system.